1. Field of the Invention
The invention relates to a method and devices for scrubbing gases.
2. Description of Related Art and Summary of the Invention
For reducing nitrogen oxides from gaseous samples, catalytically active spinels and/or spinel-type compositions have been used with the general chemical formulae: EQU metal (I) metal (II) Al.sub.2 O.sub.4 and/or EQU metal (I) oxide.metal (II) oxide.Al.sub.2 O.sub.3.
However, calculations of the catalytic effect of these compositions assumes they will be used above approximately 150.degree. C. If these compositions are used to scrub exhaust, especially from internal combustion engines, only a slight reduction in nitrogen oxides levels is detected in the starting phase, or immediately after the engine is started, during the period when the temperature of the above-mentioned compositions is below approximately 150.degree. C. Consequently, there is increased emission of nitrogen oxides during the starting phase if these compositions are used in vehicle exhaust systems. In addition, these compositions exhibit reduced effectiveness in removing nitrogen oxides when water and/or steam are present.
In one supported catalyst previously described, an aluminum oxide catalyst containing silver is used to decompose nitrous oxide. The method for its manufacture is discussed in PCT application WO 94/02244. The application of that catalyst to the removal of nitrogen oxides, especially at temperatures below 200.degree. C., is not discussed in WO 94/02244.
A goal of the present invention is to develop methods, devices, and materials for use in reducing the total content of gases, especially nitrogen oxides, optionally in the presence of water and/or steam, from certain emissions, preferably at temperatures below 200.degree. C. This particular goal is achieved through the methods and devices of this invention by employing an intermediate storage medium with gas flow channels in its interior. The intermediate storage medium is composed of a porous storage material for storing nitrogen oxide at a particular temperature range and a supporting material. The intermediate storage medium may be a component of a gas scrubbing device, such as part of a motor vehicle exhaust system.
Advantageously, a supporting material is chosen that simultaneously exhibits a storing effect for hydrocarbons in the nitrogen oxide storing temperature range of the storage material and a catalytically-active effect for nitrogen oxides above a material-specific response temperature. In choosing a supporting material and preparing the compounds and devices of this invention, copending U.S. applications Ser. Nos. 08/764,465 now U.S. Pat. No. 5,905,056, 08/764,464, 08/764,461, and 08/763,535, now abandoned each filed Dec. 12, 1996, can, be referred to and are specifically incorporated herein by reference.
Choosing a supporting material exhibiting a hydrocarbon storing effect is especially advantageous when the method is used or this intermediate storage medium is used with internal combustion engines operated on hydrocarbons, preferably motors or gasoline or diesel engines. Generally, the stored hydrocarbons are released from the supporting material at temperatures above the response temperature at which catalytic reduction of the nitrogen oxides takes place with the aid of hydrocarbons. Preferably, in this regard, the response temperature of the supporting material lies in the vicinity of or below the upper limit of the nitrogen oxide storing temperature range of the storage material, since reduction of nitrogen oxides increases when the storage effect of the storage material decreases.
The supporting material selected for use in an intermediate storage medium may comprise spinels and/or spinel-type compositions according to the formal chemical formula Cu.sub.x Me.sub.y Al.sub.z O.sub.4, since compositions of this type exhibit a significant catalytic effect with respect to nitrogen oxides at higher temperatures (above approximately 150.degree. C.). In this case, not only the nitrogen oxides but also the hydrocarbons are extracted from the gas at lower temperatures, so that the total emission output is reduced.
In an exhaust line of an internal combustion engine, the intermediate storage medium is logically used as a filter body that simultaneously causes the storage of NOx and optionally hydrocarbons at temperatures below the response temperature of the supporting material and causes the oxidation of hydrocarbons and the reduction of nitrogen oxides above the response temperature, with the release of the stored hydrocarbons and nitrogen oxides to the gas advantageously taking place in this temperature range. Oxidation and reduction as well as storage of the hydrocarbons are performed by the supporting material that may have additional materials and/or elements besides those mentioned, while the intermediate storage of NOx is performed by the storage material mixed with the support material.
Further logical embodiments can be derived from the descriptions below. In addition, the invention is explained in greater detail with reference to the embodiments shown in the drawings.
Other objects, advantages and novel features of the invention will be apparent to one skilled in the art from the following detailed description of the invention and in conjunction with the accompanying drawings.